ORIGIN AND EVOLUTION OF LIFE Notes

Origin AND EVOLUTION of life

There are different theories about the origin of life on the earth.

Theory of special creation

According to this theory, life was created by the supernatural power of the god. This theory is based on religious belief.

 According to Hindu mythology, the god of creation is Brahma. He created the universe and human beings.

 According to Christian thought, God created the universe, the earth, the plants, the animals, and the man in seven days.

 

Theory of spontaneous generation
 According to this theory, life arises spontaneously from non-living beings. For example, the frog, the toad, the snake come from the mud. Insects and flies came from fruit juices and microorganisms in the air and water.

Theory of Biogenesis
According to this theory, life originated from preexisting life but not spontaneously. Francis Redi and Louis Pasteur prove this

 Reddi's Experiment (1688 AD) 
He was an Italian biologist. He kept dead fish, snakes, and meat in wide-necked flasks and left them open. He replaced the same things in other bottles and sealed the mouth. He observed small white worms and larvae in open vials but absent in sealed vials. He concluded that life can only come from preexisting life, which means part of the preexisting life went into open mouth vials, so worms and larvae developed into mouth vials. opened. 

Louis Pasteur's Experiment (1862 AD)
·       He was a French microbiologist. He conducted an experiment to prove the theory of biogenesis.
·       He poured the nutrient solution into the flask.\
·       He bent the neck of the flask into an S shape using heat.
·       The liquid was boiled for several minutes so that bacteria were killed or expelled from the flask.
·       The solution was then cooled. Dust particles got trapped in the bend of the tube. The solution remained sterile for months.

•  When the neck broke and the balloon was tilted, the liquid came into contact with dust or microorganisms. Within days, bacterial colonies or mold appeared.

Biochemical Theory (Modern Theory) 
The scientific theory proposed by A.I. Oparin in 1923 and J.B.S. Hal Dane in 1928 is called a biochemical theory or modern theory of Oparin and Haldane theory. According to this theory, life is born from a series of biochemical reactions.

Origin of earth 

About 4600 million years ago the earth was born. The earth was a melted fireball with a temperature between 4000 and 8000 ºC. The earth began to cool and the volume decreased. In the process, the heavier elements settled towards the center forming the earth's crust and the lighter elements remained on the outer surface forming the atmosphere. In the atmosphere, there were free atoms such as C, H, O, N, etc. The free atoms have combined to form molecules such as O2, N2, and H2.O+O….. O₂   /    N+N…. N₂    /   H+H...  H₂

 

Origin of Inorganic Molecules 
Then these molecules started to combine to form inorganic molecules.

H₂+O₂……    H₂O                  H₂+N₂…..   NH₃

C+O₂…….. CO/CO₂              C+H₂…... CH₄

 

Origin of Simple Organic Molecules 
The inorganic molecules combined to form simple organic molecules.

H₂O + CH₄……….     Sugar/Fatty Acid/Glycerol

H₂O+NH₃+CH₄…....   Amino Acid/Nitrogen Base

 

Origin of Complex Organic Molecules
The organic molecules again combined together to form complex molecules

Sugar + Sugar…….................Starch/Glycogen/Cellulose etc.
Fatty acid + Glycerol……….…Fats
Amino Acid +Amino Acid…….Protein
Nitrogen Base + Sugar + Phosphoric Acid….…Nucleotides'
Nucleotide + Nucleotide …... Nucleic Acid (DNA/RNA)

On further cooling water vapor condensed into rain and thunderstorm occurred. UV rays and electric discharge enhanced these reactions. The nucleic acid combined with protein and nucleoprotein particle formed. It was a self-replicating particle and was the first living particle 'the virus'.

Origin of Primitive Cell
The complex organic molecules aggregated together forming a colloidal mass called coacervate. The nucleic acid controlled the activities of coacervate and the fats formed a layer around it. This structure is called Eobiont. It was the first cell-like structure. The eobiont absorbed food materials from the surrounding. It was heterotroph, prokaryotic cell, and present-day bacteria.

Origin of Heterotrophs and Autotrophs
Later on, some of the eobionts became able to make food by using chemical energy. So they became chemoautotroph.

CO₂+H₂S………  C₆H₁₂O₆ + H₂O + S

Some other eobionts developed chlorophyll molecules, which trapped solar energy to manufacture food from CO₂ and H₂O. They became photoautotrophs.

CO₂+H₂S…………   C₆H₁₂O₆ + H₂O + O₂

In this way, photosynthetic organisms were originated on the earth. On other hand, the heterotrophs modified and different types of animals evolved. They started to consume autotrophs and used O₂ to break down food material to release energy.